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New Member

WAN Termination Design Question. Two ISP's and Two Routers.

Hi All,

We have two different ISP links at our office which are as of now terminated on two different routers. Our Primary ISP has a T3 connection terminated on Router-2 and our backup ISP has a slow T1 connection termination at Router-1. The Primary ISP is running BGP and we are advertising our Public AS to the ISP and only taking the default route from them.

My concern is that if my Router 2 fails then we loose link to our Primary ISP and over the slow link the Internet services won't work fine.

So I proposed a design wherein we terminate both the ISP's on both the routers, however our ISP denied that this design is not feasible, however I fail to understand why.

Does this design (New Setup) look fine? Are there any limitations why I cannot do this?

Network Plan





Everyone's tags (1)
New Member

Hi, Im a newbie, but i find

Hi, Im a newbie, but i find this very complex diagram (new setup). but if I may ask, isnt it that isp suppose to have only single terminal going to 1 respective router? unless if they(isp) provide a dual wan router with failover 

New Member

So the Primary ISP has a MUX

So the Primary ISP has a MUX at our site from which we have copper connection to the Wan Router-2.

Using a switch, I can connect that Copper connection to Both Router 1 and Router 2.


Hall of Fame Super Silver

An important question is what

An important question is what size subnet does the primary ISP give you? For this type of connection most ISP will use /30 which gives 2 usable addresses. But for your new design you need 3 addresses and would require a /29 subnet. This might be one reason the ISP says your new design is not feasible.


There are also questions about how you are currently doing BGP and how you would have to change it if each router is connected to both ISP. Would you plan for each router to run BGP with both ISPs? Are you currently using the same AS number for BGP on both of your routers?


There is also perhaps some question about the media. T1 and T3 are most often run as point to point connections. You are now talking about making the T1 and T3 into multipoint connections.


And the biggest question is how much improvement you will get with the new design. What are you protecting against if you connect T3 to both your routers? Essentially you are protecting agains a failure of the connection from the mux to your devices. In my experience most often the problem is an issue in the mux, or an issue on the circuit between the mux and the ISP, or a problem in the ISP network. Your new design gives you no protection against any of these.






New Member

Exactly they are using a /30

Exactly they are using a /30 subnet and thta's why I asked the ISP to increase them to increase the subnet to /29, they agreed to increase the subnet but told me that this would not work which is why i got confused wherin I do not see any techinical challenge in this setup.

So the main purpose of the design is to build redundancy for Router-2 which currently has the Primary ISP.

In case the Router-2 fails as of now, because the secondary link does not serve us good, from Router-1 we would still be able to connect through BGP to our Primary ISP

Also currently only the Primary ISP has BGP running. and Yes we would use the same AS on both the routers.
 We are only getting the Default route using BGP and going out using our Public Subnet.


Hall of Fame Super Silver

Ok the new design does

Ok the new design does provide some protection against failure of your router 2.


I interpreted your drawing to indicate BGP on both routers and thought it meant BGP to both providers.


I still believe that there will be issues trying to terminate the T3 on a switch and connect through the switch to both routers. Will the connection from the switch to the routers be T3 or will it be just Ethernet? How will clocking on the T3 work? Will you keep the T3 as a single channel and if so how does the switch split it to the routers? Or would you ask the provider to configure two subrate channels and send a channel to each router?


And you still have a single point of failure. Now it will be the switch and not router 2. What has been the failure rate of router 2?





New Member

So the connection to the WAN

So the connection to the WAN router from the Mux is a Copper/Ethernet connection, which I believe I can connect to the two downstream routers.

The switch I am talking about will be a Nexus 2248 Fex, connected to a pair of Nexus 5K's.

Hall of Fame Super Silver

So the connection from the

So the connection from the ISP to router 2 is on a regular Ethernet interface and not a serial interface?





New Member

Yes, It's a regular Ethernet

Yes, It's a regular Ethernet from the Mux connected to the Public Interface of the router.

New Member

RichardYes this is the plan


Yes this is the plan to have BGP with both the providers. Is there any issue with this design?

Hall of Fame Super Silver

Here are some points to

Here are some points to consider.

- on each of your routers you will run BGP. Each router will have BGP peer with both providers (EBGP) and probably also have BGP peer with your other router (IBGP).

- BGP will want to use the same AS number for you with each of the providers. Do you have an assigned AS number or will you use private AS number? Are both ISP accepting of your AS number?

- do you have your own IP address space? Or are you using IP address space assigned by the provider?

- Will you advertise your address space only to the primary ISP or to both ISP?

- You can use local preference in BGP to make one provider the primary and the other provider the backup.





New Member

Yes we have an assigned

Yes we have an assigned Public AS and a Public IP space which we use for Internet Access.


Yes Both ISP's will be accepting our Public AS, We plan to advertise our address space to both the ISP's, and yes we would want to have bgp configuration such that Primary ISP has preference over secondary.


But with all the given conditions. and our ISP giving us a /29 subnet, I believe this setup should work right? I still need to have a discussion with our ISP's Network Staff, as they said even if they give us a /29 subnet and configure two bgp neighbors on their end, this setup would not work, which I am opposing. 

Hall of Fame Super Silver

Given the additional

Given the additional information that you have given you should have the discussion with your ISP staff and see what issues they believe will prevent it from working. Please update this thread with the information learned in that discussion.





New Member

I sure will. I have set a

I sure will. I have set a discussion with them next week and will update the thread on what comes out of it.

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